In recent years, non-contact power transmission (also referred to as contactless power feed or wireless power feed) has begun to spread to supply power to electronic apparatuses. WPC (Wireless Power Consortium) was organized to promote mutual use among products of different makers and the Qi standard as an international standard was formulated by WPC.
FIG. 1 is a view showing the configuration of a wireless power feed system 100 according to the Qi standard. The power feed system 100 includes a power transmitter (Tx) 200 and a power receiver (Rx) 300. The power receiver 300 is mounted on electronic apparatuses such as a mobile phone terminal, a smart phone, an audio player, a game machine, a tablet terminal and the like.
The power transmitter 200 includes a transmission antenna 201, an inverter 204, a controller 206, and a demodulator 208. The transmission antenna 201 includes a transmitting coil (primary coil) 202 and a resonance capacitor 203. The inverter 204 includes an H bridge circuit (full bridge circuit) or a half bridge circuit, applies a drive signal S1, specifically a pulse signal, to the transmitting coil 202, and generates an electromagnetic field power signal S2 in the transmitting coil 202 by a drive current flowing through the transmitting coil 202. The controller 206 generally controls the overall operation of the power transmitter 200. Specifically, the controller 206 changes transmission power by controlling the switching frequency, switching duty ratio and phase of the inverter 204 or the supply voltage VDC for the inverter 204. The controller 206 may be implemented by hardware such as a logic circuit or an FPGA (Field Programmable Gate Array) or may be implemented by a microcomputer and a combination of a CPU (Central Processing Unit) and a software program.
In the Qi standard, a communication protocol is defined between the power transmitter 200 and the power receiver 300 so as to allow control data S3 to be transferred from the power receiver 300 to the power transmitter 200. The control data S3 is transmitted from a receiving coil 302 (secondary coil) to the transmitting coil 202 in an AM (Amplitude Modulation)-modulated form by using backscatter modulation. The control data S3 includes, for example, power control data (also referred to as a packet) indicating the amount of supply of power to the power receiver 300, data indicating unique information of the power receiver 300, and the like. The demodulator 208 demodulates the control data S3 included in a current or voltage of the transmitting coil 202. The controller 206 controls the inverter 204 based on the power control data included in the demodulated control data S3.
The power receiver 300 includes the receiving coil 302, a rectifying circuit 304, a smoothing capacitor 306, a power supply circuit 308, a modulator 310, a demodulator 312, and a controller 314. The receiving coil 302 receives the power signal S2 from the transmitting coil 202 and transmits the control data S3 to the transmitting coil 202. The rectifying circuit 304 and the smoothing capacitor 306 rectify and smooth a current S4 induced in the receiving coil 302 in response to the power signal S2 and convert the induced current S4 into a DC voltage VRECT. The power supply circuit 308 stabilizes the DC voltage VRECT and generates an output voltage VOUT. The output voltage VOUT is supplied to a load circuit (not shown).
The controller 314 is, for example, a microcontroller or a CPU and executes a software program to support power feed conforming to the Qi standard. For example, the controller 314 monitors the power supply amount received by the power receiver 300 and generates power control data (control error value) indicating a power supply amount according to the monitored power supply amount. The modulator 310 changes a coil current and a coil voltage of the transmitting coil 202 by changing the parallel resonance frequency of a receiving antenna 301 based on the control data S3 including the power control data, and transmits information.
According to the Qi standard, control data S5 can be transferred from the power transmitter 200 to the power receiver 300. The control data S5 is superimposed on the power signal S2 by FSK (Frequency Shift Keying) and is transmitted from the transmitting coil 202 to the receiving coil 302. The control data S5 may include an acknowledge (ACK) signal, a non-acknowledge (NAK) signal and the like.
An FSK modulator 220 is incorporated in the controller 206 and changes the switching frequency of the inverter 204 according to data to be transmitted. The demodulator 312 of the power receiver 300 demodulates the FSK-processed control data (also referred to as an FSK signal) S5. The above is the configuration of the power feed system 100.
The rectifying circuit 304 of a synchronous rectification type is configured with a transistor. In this case, it is necessary to prevent the rectified voltage VRECT of a RECT pin (terminal) from exceeding the breakdown voltage of the transistor. To this end, an overvoltage protection circuit is provided. For example, when the breakdown voltage of the transistor is 20 V, an overvoltage threshold VOVP is set around 16 V and the protection works when the rectified voltage VRECT exceeds the overvoltage threshold VOVP.
As a result of studies on this overvoltage protection, the present inventors have recognized the following problems. When the modulator 310 changes the parallel resonance frequency of the receiving antenna 301, the rectified voltage VRECT periodically jumps up accordingly.
The Qi standard was targeted to feed power of 5 W or less (low power) at the beginning of its formulation but was thereafter changed to a standard capable of feeding power of 5 to 15 W (medium power). In the low power, since the rectified voltage VRECT in the steady state is as low as 5 to 7 V, even when headroom for the overvoltage threshold VOVP (16 V) is sufficiently large and the rectified voltage VRECT jumps up due to the modulation of the modulator 310, the rectified voltage VRECT rarely exceeds the overvoltage threshold VOVP.
On the other hand, in the medium power, since the rectified voltage VRECT in the steady state is higher (for example, about 12 to 14 V) than the low power, the headroom for the overvoltage threshold VOVP (16 V) becomes smaller. Therefore, when the rectified voltage VRECT jumps up due to the modulation of the modulator 310, it exceeds the overvoltage threshold VOVP, making the operation unstable.
Such a problem may occur not only in the Qi standard but also in a power feed system conforming to or according to a standard formulated by PMA (Power Matters Alliance) (hereinafter referred to as a PMA standard).